1. Technical Field
The present invention relates generally to electrical circuits and in particular to transformer circuits. Still more particularly, the present invention relates to transformer circuits and designs that reduce harmonic currents in three phase applications.
2. Description of the Related Art
When powering certain types of electrical devices and/or circuits, such as a variable frequency drive (VFD) utilized to supply power to nonlinear loads, such as large pumping systems in the offshore petroleum industry, the occurrence of harmonic currents associated with the alternating current (AC) power source presents an undesirable effect in the operations of the device/circuit. The amount of harmonic currents present in a conventional VFD system (depicted in FIG. 1) is typically directly correlated to the pulse number (number of current pulses) associated with the power input to the converter sections of the VFD. In turn, with conventional VFD designs, the pulse number is equal to two times (2×) the number of converter/inverter modules within the overall drive system. This 2× pulse characteristic results from the fact that each series connected device in each of the three output legs has a corresponding device in each of the other two legs where the current pulses occur simultaneously. Additionally, the VFD systems themselves often present a nonlinear load to the AC power source, and thus manifest/exhibit a high incidence of harmonic currents in the power system.